A solid state imaging apparatus is roughly divided into a charge transfer solid state imaging apparatus represented by, for example, a CCD (Charge Coupled Device) image sensor and an X-Y addressable solid state imaging apparatus represented by, for example, a CMOS (Complementary Metal Oxide Semiconductor) image sensor. This type of solid state imaging apparatus is used in various video apparatuses such as a video camera for photographing moving images and an electronic still camera for photographing still images as an imaging apparatus therefor.
Conventionally, when the CCD image sensor is used as an imaging apparatus in a 1CCD color camera, in a so-called pixel skipping processing for skipping pixel information, for example, a method of reading out pixel information for all pixels from the image sensor and, then, skipping the pixel information in an external signal processing system is adopted. When the CMOS image sensor is used as an imaging apparatus, a method following the pixel skipping processing method in the CCD image sensor is adopted. In such a pixel skipping processing method, although an amount of information is reduced by the pixel skipping processing, since a driving frequency of the image sensor is unchangeable, power consumption is not reduced. On the contrary, this results in a load on the signal processing system.
Reasons for adopting the pixel skipping and readout method of reading out pixel information for all pixels and, then, performing the pixel skipping processing for the pixel information in the external signal processing system are, for example, as follows:    <1> It is impossible to skip pixel information while keeping a spatial arrangement and an output order of color filters;    <2> In an ordinary shift register used as selecting means for selecting pixels, the pixels are selected in order; and    <3> In the CCD image sensor, information can be read out from pixels only in an order.
In order to solve those problems, conventionally, the X-Y addressable solid state imaging apparatus, in which color filters having predetermined color coding are formed for respective pixels arranged in a matrix shape, is used. When pixel skipping and readout is designated for this X-Y addressable solid state imaging apparatus, a clock frequency of a system is changed and pixels are selected in an order corresponding to the color coding on the basis of the changed clock frequency to read out pixels signals, whereby the pixel skipping processing is performed at a stage when pixel information is read out from the pixels (see, for example, a patent document 1 (JP-A-2001-298748)).
As shown in FIG. 31, pixel skipping and readout is performed by half in a color filter arrangement having color coding of a G (green) stripe system with four rows set as a row block and four columns set as a column block. In this case, when a first row of a pixel matrix is read out, in an odd-number column block, first and second columns in the block are read out and, in an even-number column block, first and fourth columns in the block are read out. When a third row of a first row block, that is, a third row of the pixel matrix is read out, in an odd-number column block, first and fourth columns in the block are read out and, in an even-number column block, first and second columns in the block are read out (see, for example, a patent document 2 (JP-A-2000-004406)).
In the first conventional technique disclosed in the patent document 1, it is possible to skip and compress an amount of pixel information without applying a load on the signal processing system. Moreover, it is possible to reduce power consumption following a change in a clock frequency of the system. It is also possible to fix a frame rate even if an operation mode is changed. On the other hand, when pixel information is read out at intervals while subjecting the pixel information to the pixel skipping processing with a system clock set to 1/9 (a pixel skipping ratio set to 1/9), a distance between pixels to be read out increases.
When a distance between pixels is p, a Nyquist frequency fn depending on an arrangement pitch p of pixels is represented by expression (1) below.fn=1/2p  (1)Therefore, as the distance p between pixels increases, the Nyquist frequency fn decreases and patterns having a spatial frequency equal to or higher than the Nyquist frequency fn relatively increase. Thus, aliasing noise increases.
The increase in the distance p between pixels at the time when the pixel skipping ratio is increased will be explained with reference to pixels arrangements shown in FIGS. 30A to 30C. In FIGS. 30A to 30C, shaded pixels are pixels to be read out in a pixel skipping process. In FIG. 30A, a pixel arrangement at the time when all pixels are read out is shown. A distance between pixels is the same as a pixel width. In FIG. 30B, an example of a pixel arrangement at the time when pixel information is skipped at a ratio of 1/9 is shown. A distance between pixels to be read out is increased to a distance equivalent to three pixels. In FIG. 30C, an example of a pixel arrangement at the time when pixel information is skipped at a ratio of 1/25 is shown. A distance between pixels to be read out is increased to a distance equivalent to five pixels. It is seen that, when the pixel skipping ratio is increased, a distance between pixels increases, and aliasing noise increases following a decrease in the Nyquist frequency, affecting images significantly.
In the second conventional technique described in the patent document 2, when the pixel skipping processing is performed in the procedure described above, shaded pixels are pixels to be read out in FIG. 31. Consequently, for example, there are two kinds (A and B) of intervals between pixels to be read out. Since the pixels have a different spatial frequency for each pixel pitch, aliasing noise increases and a false color occurs. In the process of performing the pixel skipping processing in this way, there is a significant problem in that aliasing noise increases and false colors increase.
In short, at the time of the pixel skipping and readout, when pixels are simply skipped to be read while keeping both an order of pixel information and a spatial positional relation the same as those in all-pixel readout such that the same color arrangement of pixels is obtained, since a distance between pixels to be read out increases, the Nyquist frequency depending on an arrangement pitch decreases. Thus, patterns having a spatial frequency equal to or higher than the Nyquist frequency relatively increase, resulting in an increase in aliasing noise. In addition, in the pixel skipping and readout method in which pixels to be read out have two or more distances between pixels, since the pixels have a different spatial frequency for each pitch, aliasing noise increases.
The invention has been devised in view of the problems described above and it is an object of the invention to provide a solid state imaging apparatus and a driving method therefor that make it possible to reduce aliasing noise by increasing a pixel area simulatively.